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Register a new userEtching of semiconductors and metals by the photonic jet with shaped optical fiber tips
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Authors
Robin Pierron
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The etching of semiconductors and metals by a photonic jet (PJ) generated with a shaped optical ber tip is studied. Etched marks with a diameter of 1 micron have been realized on silicon, stainless steel and titanium with a 35 kHz pulsed laser, emitting 100 ns pulses at 1064 nm. The selection criteria of the ber and its tip are discussed. We show that a 100/140 silica ber is a good compromise which takes into account the injection, the working distance and the energy coupled in the higherorder modes. The energy balance is performed on the basis of the known ablation threshold of the material. Finally, the dependence between the etching depth and the number of pulses is studied. Saturation is observed probably due to a redeposition of the etched material, showing that a higher pulse energy is required for deeper etchings.
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We demonstrate the key role of the laser injection into a multimode fiber to obtain a photonic jet (PJ). PJ, a high concentrated propagating beam with a full width at half maximum smaller than the diffraction limit, is here generated with a shaped optical fiber tip using a pulsed laser source (1064~nm, 100~ns, 35~kHz). Three optical injection systems of light are compared. For similar etched marks on silicon with diameters around 1~$mu$m, we show that the required ablation energy is minimum when the injected light beam is close to the fundamental mode diameter of the fiber. Thus, we confirm experimentally that to obtain a PJ out of an optical fiber, light injection plays a role as important as that of the tip shape, and therefore the role of the fundamental mode in the process.
Although, poly(dimethylsiloxane) (PDMS) is a widely used material in numerous applications, such as micro- or nanofabrication, the method of its selective etching has not been known up to now. In this work authors present two methods of etching the pure, additive-free and cured PDMS as a positive resist material. To achieve the chemical modification of the polymer necessary for selective etching, energetic ions were used. We created 7 um and 45 um thick PDMS layers and patterned them by a focused proton microbeam with various, relatively large fluences. In this paper authors demonstrate that 30 wt% Potassium Hydroxide (KOH) or 30 wt% sodium hydroxide (NaOH) at 70 oC temperature etch proton irradiated PDMS selectively, and remove the chemically sufficiently modified areas. In case of KOH development, the maximum etching rate was approximately 3.5 um/minute and it occurs at about 7.5*10^15 ion*cm-2. In case of NaOH etching the maximum etching rate is slightly lower, 1.75 um/minute and can be found at the slightly higher fluence of 8.75*10^15 ion*cm-2. These results are of high importance since up to this time it has not been known how to develop the additive-free, cross-linked poly(dimethylsiloxane) in lithography as a positive tone resist material.
We report on the rst evidence of direct micropeak machining using a photonic jet (PJ) with nanosecond laser pulses. PJ is a high concentrated propagative light beam with a full width at half maximum (FWHM) smaller than the diraction limit. In our case, PJs are generated with a shaped optical ber tip. Micropeaks with a FWHM of around 1 $mu$m, a height until 590 nm and an apex radius of 14 nm, were repeatability achieved on a silicon wafer. The experiments have been carried out in ambient air using a 100/140 multimode silica ber with a shaped tip along with a 35 kHz pulsed laser emitting 100 ns pulses at 1064 nm. This study shows that the phenomenon occurs only at low energies, just under the ablation threshold. Bulk material appears to have moved around to achieve the peaks in a selforganized process. We hypothesize that the matter was melted and not vaporized; hydrodynamic ow of molten material governed by surfacetension forces may be the causes. This surface modication has many applications. For example, this paper reports on the decrease of wettability of a textured silicon wafer.
In this work authors present for the first time how to apply the additive-free, cured PDMS as a negative tone resist material, demonstrate the creation of PDMS microstructures and test the solvent resistivity of the created microstructures. The PDMS layers were 45 um and 100 um thick, the irradiations were done with a focused proton microbeam with various fluences. After irradiation, the samples were etched with sulfuric acid that removed the unirradiated PDMS completely but left those structures intact that received high enough fluences. The etching rate of the unirradiated PDMS was also determined. Those structures that received at least 7.5*10^15 ion*cm-2 fluence did not show any signs of degradation even after 19 hours of etching. As a demonstration, 45 um and 100 um tall, high aspect ratio, good quality, undistorted microstructures were created with smooth and vertical sidewalls. The created microstructures were immersed into numerous solvents and some acids to test their compatibility. It was found that the unirradiated PDMS cannot, while the irradiated PDMS microstructures can resist to chloroform, n-hexane, toluene and sulfuric acid. Hydrogen fluoride etches both the unirradiated and the irradiated PDMS.
Despite the fact that the resolution of conventional contact/proximity lithography can reach feature sizes down to ~0.5-0.6 micrometers, the accurate control of the linewidth and uniformity becomes already very challenging for gratings with periods in the range of 1-2 {mu}m. This is particularly relevant for the exposure of large areas and wafers thinner than 300{mu}m. If the wafer or mask surface is not fully flat due to any kind of defects, such as bowing/warpage or remaining topography of the surface in case of overlay exposures, noticeable linewidth variations or complete failure of lithography step will occur. We utilized the newly developed Displacement Talbot lithography to pattern gratings with equal lines and spaces and periods in the range of 1.0 to 2.4 {mu}m. The exposures in this lithography process do not require contact between the mask and the wafer, which makes it essentially insensitive to surface planarity and enables exposures with very high linewidth uniformity on thin and even slightly deformed wafers. We demonstrated pattern transfer of such exposures into Si substrates by reactive ion etching using the Bosch process. An etching depth of 30 {mu}m or more for the whole range of periods was achieved, which corresponds to very high aspect ratios up to 60:1. The application of the fabricated gratings in phase contrast x-ray imaging is presented.
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